Optical measurement systems often separate polarization components of a light beam and use the separated components for different purposes. An interferometer, for example, can separate and use one polarization component for a measurement beam that reflects from an object being measured, while the other orthogonal polarization component forms a reference beam that is compared with the measurement beam. An advantage of using polarization components is that the two component beams can be collinear and share the same common mode effects when desired and can be separated or recombined using polarizing beam splitters. However, the polarizations of the component beams in such measurement systems must generally be carefully controlled to avoid polarization changes that mix the component beams.
One result of polarization mixing in interferometers is commonly referred to as cyclic errors or cyclic nonlinearities. U.S. Pat. Nos. 4,930,894 and 4,693,605, for example, describe interferometers in which polarization change of a measurement beam or a reference beam can cause polarization leakage between the reference beam and the measurement beam, resulting in cyclic nonlinearities or measurement errors. Sources of polarization change in these interferometers include imperfect polarizing coatings in polarizing beam splitters, imperfect retardation plates that fail to change polarizations in the desired manner, and retroreflectors such as cube corner reflectors.
A retroreflector such as a solid cube corner reflector generally returns a reflected beam that is parallel to the incident beam, regardless of the angle of the incident beam. This property makes retroreflectors useful for a wide variety of optical systems. However, uncoated solid cube corner reflectors generally do not preserve the polarization state of the incident beam. A reflective coating (e.g., a silver coating) can be used on a solid cube corner reflector to mitigate or minimize the polarization change, but residual polarization changes from a coated cube corner reflector can still be a limiting factor in the precision or accuracy of measurement systems that separate polarization components.
Precision optical systems are thus desired that can employ retroreflectors but avoid unwanted mixing of polarization components.